A conventional curtain type decorative light string set is composed of a main electrical wire as a supporting means and a plurality of short light strings which are hung separately from the main wire as shown in FIG. 1, that arrangement forms a decorative light string set forming a curtain of lighting bulbs. The structure of this prior art includes several connecting bulb holders a1 on the main electrical wire to engage with the short light strings to be hung therefrom. Since the short light strings can possibly come loose because of the weight of the strings or can be pulled by accident, those light strings may become unlit. If the connecting bulb holder a1, located near to a plug of this light string set, is broken, it might cause the whole light string set to become unlit, as the bulb holder a1 is connected in series with the bulb holder a.